TECHNICAL FIELD
[0001] This patent generally relates to turbofan engines, and in particular, to a fan case
for a turbofan engine having a structure to retain a fan blade or fan blade portion
following a fan blade out event.
BACKGROUND
[0002] Turbofan engines operate with an high level of safety, reliability and efficiency,
which is a credit to the engine designer who is challenged to design an engine to
operate under numerous conditions and to address any contingency. One such contingency
event is known as a fan blade out event.
[0003] The fan is a vital component of a turbofan engine such as those that are commonly
found on aircraft. The fan will have fan blades that extend radially outwardly from
a central hub or disk. Fan designs differ primarily in the manner in which the blades
are structurally tied or connected to the disk. In one fan design type, a dovetail
connection is used to connect the fan blade and an associated shank to the disk. In
another fan design type, the fan blade airfoil itself, without a shank, is welded
to the disk. This later design is sometimes referred to as a bladed disk (or BLISK)
fan, as the blades and disk form an integrated unit.
[0004] The fans operate at high rotational speed, and the fan blades themselves experience
significant operating stresses, particularly in a radial direction extending away
from the disk. In the fan blade out event, the fan blade or a portion of the fan blade
separates from the disk and is discharged into the engine case. The design challenge
is to ensure that fan blade fragments and other detritus are contained by the engine
case during a fan blade out event.
[0005] In a typical fan blade out event, the fan blade separates at a minimum cross-sectional
thickness region adjacent the disk. The result is the release of the fan blade airfoil
part and the shank part, if a dovetail design, from the disk. The released fan blade
interacts with at least the first trailing blade and the surrounding engine case.
[0006] In the case of a dovetail design, because of the presence of the platform and shank,
the released blade interacts with the first trailing blade and the containment case
simultaneously. Because of the blade profiles, the first trailing blade tends to draw
the released blade aft-wards, i.e., into the engine axially. Whereas the impact force
on the released blade due to its interaction with the containment case tries to push
the released blade forward due to the conical shape of the containment case. In a
dovetail because of the significant interaction of the released blade with first trailing
blade, the released blade tends to move in a direction into the engine.
[0007] In aircraft engines with a BLISK type fan blade design, the fan blades are frictionally
welded to the disk at the root. Hence there is no shank present at the root of the
fan blade as would be found in a dovetail type fan blade design. In a fan blade out
event in an engine with a BLISK fan blade design, it is more likely an airfoil part
of the blade will separate from the disk than for the disk itself to separate or disintegrate.
It has been observed that as much as 80% or more of the full blade airfoil may separate
in a fan blade out event of a BLISK fan.
[0008] The separated airfoil portion of a BLISK fan blade, even as much as the entire fan
blade, has less mass and therefore less inertia than a dovetail design fan blade,
which includes the shank and attaching platform. Moreover, the absence of a shank
and attaching platform from the BLISK fan blade moves the center-of-gravity (CG) of
the released airfoil portion radially outwardly from the disk. A result of these differences
is that in a fan blade out event in a BLISK fan engine, the released blade part to
first trailing blade interaction occurs much later after blade release. By the time
of interaction with the first trailing blade, which tends to draw the released blade
into the engine, the desired direction, as much as or more than 50% of the released
blade part will have engaged the engine case. Engagement of the blade part with the
engine case tends to move the blade part toward the engine inlet and potentially out
of the engine case.
[0009] Therefore, it is desirable to provide an engine case design for turbofan engines
that has the effect of retaining fan blade parts within the engine case in a fan blade
out event. This may be accomplished by an engine case design that tends to direct
fan blade parts into the engine after a blade out event. Other desirable features
and characteristics of the herein described embodiments will become apparent from
the subsequent detailed description and the appended claims, taken in conjunction
with the accompanying drawings and the foregoing technical field and background.
SUMMARY
[0010] A turbofan engine has an engine case with an inlet and an interior. A bearing is
disposed on a portion of the engine case. A fan is disposed within the engine and
rotates with operation of the engine. The fan includes a plurality of fan blades,
and each fan blade is secured at a first end to a disk. A second end of each fan blade
is disposed adjacent to the bearing. The engine case portion is oriented relative
to the fan such that a reaction force of a fan blade impacting the bearing is directed
axially or away from the inlet until a first trailing blade interacts with the fan
blade.
[0011] A case for a turbofan engine includes a case structure and bearing configured and
cooperate to effect a primary load bearing member such that the engagement of a fan
blade or portion thereof following a fan blade out with the first trailing blade becomes
significant.
[0012] A method of containing a fan blade or portion thereof and collateral structures that
may be released in a fan blade out event may include retarding a radial outward movement
of the released blade portion after the blade out event until there is an interaction
of the released blade portion with a first trailing blade.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The exemplary embodiments will hereinafter be described in conjunction with the following
drawing figures, wherein like numerals denote like elements, and wherein:
FIG. 1 is a graphic cross-sectional illustration of an inlet portion of a turbine
engine in accordance with herein described exemplary embodiments;
FIG. 2 is a graphic depiction of a blade disk (BLISK) fan;
FIG. 3 is a graphic cross-sectional illustration of an inlet portion of a turbine
engine in accordance with an alternate exemplary embodiment;
FIG. 4 is a graphic cross-sectional illustration of an inlet portion of a turbine
engine in accordance with an alternate exemplary embodiment; and
FIG. 5 is a graphic cross-sectional illustration of an inlet portion of a turbine
engine in accordance with an alternate exemplary embodiment.
DETAILED DESCRIPTION
[0014] Embodiments of the subject matter described herein provide an engine case design
for turbofan engines that has the effect of retaining fan blade parts within the engine
case in a fan blade out event by directing fan blade parts into the engine after a
blade out event.
[0015] Referring to FIGs. 1 and 2, a turbine engine 10 includes an engine case 12 with an
inlet 14 and an interior 16. A bearing 20 is disposed on a case portion 18 of the
engine case 12. A fan 24 is disposed within the engine case 12 and rotates with operation
of the engine 10 in a known manner. The fan 24 includes a plurality of fan blades
(one illustrated as fan blade 26), and each fan blade 26 is secured at a first end
28 to a disk 30. A second end 32 of each fan blade is disposed closely adjacent to
a surface 34 of the bearing 20. The case portion 18 includes surface 22 that is oriented
relative to the fan 24 such that a reaction force of the fan blade 26 or a portion
of the fan blade 26 impacting the bearing 20 during a fan blade out event is directed
radially inward toward a centerline
"c/
l" of the engine 10. Because there is less axial force coming from the interaction with
case onto the blade, the axial motion of the blade is relatively away from the inlet
14 and toward the interior 16.
[0016] In an exemplary embodiment, the bearing 20 has a non-uniform thickness or cross-section
from the inlet 14 to the interior 16. As depicted in FIG. 1, the bearing 20 tapers
from a first edge 36 adjacent the inlet 14 to a second edge 38 toward the interior
16. The case portion 18 is made cylindrical. That is, the surface 22 of the portion
18 of the case 12 is substantially parallel to the engine centerline,
c/
l, and in this regard, the surface 22 of the case portion 18 forms a right, circular
cylinder. The bearing 20 by virtue of the taper provides a narrowing of the engine
inlet 14 toward the interior 16 of the engine 10.
[0017] With continued reference to FIG. 1, in a fan blade out event, the fan blade 26 or
a fan blade portion (not depicted) separates from the disk 30 at or near the first
end 28. Upon release, the fan blade 26 will move radially outwardly from the centerline
c/
l and encounters the bearing 20, which is supported by the case portion 18 along the
surface 22. A force, FN, is imparted on the fan blade 26. Because the surface 22 of
the case portion 18 is parallel to the centerline
c/
l, the force FN is primarily directed radially inward back toward the centerline
c/
l (e.g., F
y), with little or no axial component (e.g., F
x). In particular, the surface 22 is oriented such that the force FN is radially directed
toward the centerline with no significant axial component being directed toward the
inlet 14. This reduced the likelihood that the fan blade 26 will move toward the inlet
14 and outward of the engine case 12.
[0018] Still referring to FIG. 1, surrounding the case portion 18 is a containment member
40 and disposed between an inner surface 42 of the containment member 40 and an outer
surface 44 of the engine case 12 in the area of portion 18 is an energy absorbing
material 46. The energy absorbing material may be a polymeric honeycomb, or metallic
honeycomb, and the containment member 40 may likewise be made of Kevlar fiber or may
be made of carbon fiber. Alternatively, the containment member may be made of aluminum,
aluminum alloys or other metallic or polymeric materials and combinations thereof.
The energy absorbing material material 46 and containment member 40 cooperate to supplement
the engine case 12 to contain the fan blade 26 during a fan blade out event. However,
by design, the energy absorbing material 46 deflects and compresses in order to absorb
and dissipate energy. On its own, therefore, the engine case 12, energy absorbing
material 46 and containment member 40 only partially restrict radial outward movement
of the fan blade 26 during a blade out event. To retard, e.g., delay and/or limit,
at least at the initial moment of the fan blade out event, radial outward movement
of the fan blade 26, the bearing 20 is made of an abradable but stiff material, such
as aluminum, aluminum alloys, other metallic alloys or equivalent materials. It is
only necessary to briefly retard the radial outward movement of the fan blade 26 following
a fan blade out event until the first following blade 50 (see, Fig. 2) of the fan
24 interacts with the fan blade 26. The configuration of the fan blades 26 is such
that the interaction of the fan blade 26 and the first following fan blade tends to
cause an axial movement of the fan blade 26 toward the interior 16 of the engine 10.
Thus, it is evident that the structure of the bearing 20 and the case portion 18 cooperate
to provide a primary interaction of the fan blade 26 with the first following fan
blade after a fan blade out event, and to further limit movement of the fan blade
26 or fan blade portion toward the inlet 14.
[0019] Referring to FIG. 3, the case portion 18 includes a leading and trailing chamfer
52 and 54 extending between the surface 22. The surface 22 is still arranged parallel
to the centerline
c/
l. The bearing 20 has an pseudo-hexagonal cross-section that is thinner toward a first
edge 56 adjacent the inlet 14 and thicker at a second edge 58 adjacent the interior
16. In accordance with this alternative embodiment, the bearing 20 and the case portion
18 cooperate to provide a primary interaction of the fan blade 26 with the first following
fan blade 50 after a fan blade out event, and to further limit movement of the fan
blade 26 or fan blade portion toward the inlet 14.
[0020] Referring to FIG. 4, the case portion 18 is formed with a hemispherical surface 60.
The bearing 20 is ovoid, and the ovoid is thinner toward a first edge 62 adjacent
the inlet 14 and thicker at a second edge 64 adjacent the interior 16. In accordance
with this alternative embodiment, the bearing 20 and the case portion 18 cooperate
to provide a primary interaction of the fan blade 26 with the first following fan
blade 50 after a fan blade out event, and to further limit movement of the fan blade
26 or fan blade portion toward the inlet 14.
[0021] Referring to FIG. 5, the case portion 18 is formed with a first angled surface 66
and a second angled surface 68. The bearing 20 has a triangular cross-section 70 that
fills the cross-section defined by the surfaces 66 and 68, and has a surface 72 parallel
to the centerline
c/
l. In accordance with this alternative embodiment, the bearing 20 and the case portion
18 cooperate to provide a primary interaction of the fan blade 26 with the first following
fan blade 50 after a fan blade out event, and to further limit movement of the fan
blade 26 or fan blade portion toward the inlet 14.
[0022] In accordance with the herein described embodiments, a turbofan engine includes an
engine case with an engine case structure and a bearing to ensure that a fan blade
or portion thereof and collateral structures that may be released in a fan blade out
event are retained within the engine case. In one exemplary embodiment, the radial
movement of the released portion of a fan blade is retarded, e.g., delayed or limited,
sufficiently to cause it to engage and interact primarily with a first trailing blade.
Because of the profile of the fan blades, the interaction of the released blade portion
with the first trailing blade tends to direct the released blade portion axially inward
relative to an inlet of the turbine engine and into the engine case. The released
blade portion is thereby first directed into the engine case before the inertia of
the released blade portion causes it to travel radially outward and engage the containment
case and a containment structure. The net effect is that axial outward motion, relative
to the inlet of the turbine engine is reduced.
[0023] To achieve this result, the case structure and bearing are configured and cooperate
to effectively change the primary load bearing member, i.e., the primary resistance
to the movement of a fan blade or portions thereof following a fan blade out event,
such that first primary engagement of the fan blade or portion thereof following a
fan blade out event is with the first trailing fan blade. The bearing is abradable
and may be made of aluminum, aluminum alloys, other metallic alloys or equivalent
materials. The bearing may have a non uniform cross-section, and in at least one of
the herein described embodiments, tapers from a thicker section toward the engine
interior to a thinner section adjacent engine inlet. The bearing further may be made
to be stiffer than surrounding, energy absorbing containment structures to retard
axial movement of the released fan blade or released fan blade portion. This ensures
that the interaction of the released fan blade or fan blade portion with the first
trailing fan blade begins early and lasts longer, thereby effectively reducing motion
of the released fan blade or fan blade portion toward the engine inlet.
[0024] A method of containing a fan blade or portion thereof and collateral structures that
may be released in a fan blade out event may include retarding, e.g., reducing and/or
delaying, a radial outward movement of the release blade portion after the blade out
event until there is an interaction of the released blade portion with a first trailing
blade. The reducing/delaying of the radial outward movement of the released blade
portion may be accomplished by providing a bearing within the engine case adjacent
tips of the fan blades. Providing the bearing may further include retaining the bearing
by the engine case or by a retaining portion of the engine case. The retaining portion
of the engine case may be made by providing a cylindrical structure within the engine
case and engaging the bearing at least in a radial direction relative to a centerline
of an engine.
[0025] The foregoing detailed description is merely exemplary in nature and is not intended
to limit the application and uses. Furthermore, there is no intention to be bound
by any expressed or implied theory presented in the preceding technical field, background,
brief summary or the detailed description. It should be understood that throughout
the drawings, corresponding reference numerals indicate like or corresponding parts
and features. As used herein, the term system or module may refer to any combination
or collection of mechanical systems and components and/or other suitable components
that provide the described functionality.
[0026] Embodiments may be described herein in terms of functional and/or logical block components
and various processing steps. It should be appreciated that such block components
may be realized by any number, combination or collection of mechanical components
configured to perform the specified functions. Those skilled in the art will appreciate
that the herein described embodiments may be practiced in conjunction with any number
of mechanical components and systems, and that the systems described herein are merely
exemplary.
[0027] For the sake of brevity, conventional components and techniques and other functional
aspects of the components and systems (and the individual operating components of
the systems) may not be described in detail herein. Furthermore, the connecting lines
shown in the various figures contained herein are intended to represent example functional
relationships and/or physical couplings between the various elements. It should be
noted that many alternative or additional functional relationships or physical connections
may be present in an embodiment of the invention.
[0028] While at least one exemplary embodiment has been presented in the foregoing detailed
description, it should be appreciated that a vast number of variations exist. It should
also be appreciated that the exemplary embodiment or exemplary embodiments are only
examples, and are not intended to limit the scope, applicability, or configuration
of the disclosure in any way. Rather, the foregoing detailed description will provide
those skilled in the art with a convenient road map for implementing the exemplary
embodiment or exemplary embodiments. It should be understood that various changes
can be made in the function and arrangement of elements without departing from the
scope of the disclosure as set forth in the appended claims and the legal equivalents
thereof. Accordingly, details of the exemplary embodiments or other limitations described
above should not be read into the claims absent a clear intention to the contrary.
1. A turbofan engine comprising:
an engine case with an inlet and an interior of the engine;
a bearing disposed on a portion of the engine case;
a fan is disposed within the interior and rotates with operation of the engine, the
fan having a plurality of fan blades, each fan blade is secured at a first end to
a disk, and a second end of each fan blade is disposed adjacent to the bearing, wherein
the portion is oriented relative to the fan such that a force on a fan blade impacting
the bearing directs the fan blade axially inward or away from the inlet.
2. The turbofan engine of claim 1, the portion being oriented substantially parallel
to a centerline of the engine.
3. The turbofan engine of claim 2, the bearing having a non-uniform thickness or cross-section
from the inlet to the interior.
4. The turbofan engine of claim 1, the portion being one of hemispherical and ovoid.
5. The turbofan engine of claim 1, the portion including a first angled surface and a
second angled surface.
6. The turbofan engine of claim 6, the bearing having a surface adjacent the second ends
of the plurality of fan blades that is substantially parallel to a centerline of the
engine.
7. The turbofan engine of claim 1, the bearing having a non-uniform thickness or cross-section
from the inlet to the interior.
8. The turbofan engine of claim 1, further comprising a containment structure surrounding
the engine case in a region of the bearing.
9. An engine case for a turbofan engine according to any of the foregoing claims.
10. A method of containing a fan blade or portion thereof and collateral structures that
may be released in a fan blade out event within a turbofan engine comprising:
retarding a radial outward movement of a released blade portion after the fan blade
out event; and
interacting the released blade portion with a first trailing blade.